Method of using a gas sensor

ABSTRACT

A method of measuring oxygen and/or the air-to-fuel lambda ratio and hydrocarbons and/or carbon monoxide in gas mixtures using a gas sensor is provided. To reliably measure a plurality of gaseous components, the sensor is provided with a reference electrode representing a constant oxygen partial pressure, an oxygen ion-conducting solid electrolyte, and at least two measuring electrodes, the measuring electrodes and the reference electrode being mounted directly on the solid electrolyte and having electrical leads for connection and for take-away of electrical measurement signals. The solid electrolyte is constructed with a measurement gas side exposed to the gas mixture and a reference gas side separated from the gas mixture. The system of electrodes has the reference electrodes on the reference gas side and at least two measuring electrodes on the measurement gas side, and is so constructed that one of the reference electrodes is assigned to at least one measuring electrode, which forms the anode of this electrode pair. The pair of electrodes is adapted for the application pumping oxygen, and the system simultaneously transmits at least two measurement signals, which correspond to different gaseous components of the gas mixture.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of co-pending application Ser. No.09/312,184 filed May 14,1999 entitled GAS SENSOR.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of International ApplicationPCT/EP98105823, filed Sep. 14, 1998, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a gas sensor for measurement of oxygenand/or the air-to-fuel lambda ratio and hydrocarbons and/or carbonmonoxide in gas mixtures. The gas sensor has a reference electroderepresenting a constant oxygen partial pressure, an oxygenion-conducting solid electrolyte, and at least two measuring electrodes,the measuring electrodes and the reference electrode being mounteddirectly on the solid electrolyte and having electrical leads forconnection and for take-away of the electrical measurement signals. Theinvention also relates to applications for the gas sensor and to ameasurement method.

A gas sensor of a this type is known, e.g., from German published patentapplication DE 195 34 918 A1. The sensor therein has two electrodesconstructed as mutually engaging comb structures (see FIG. 1), which arearranged on the side of the solid electrolyte facing the gas beingmeasured (hereinafter “measurement gas”), and a reference electrode isprovided opposite thereto on the reference air side. That invention isdirected mainly to a reliable seal to ensure that no effects are exertedon the operation and performance of the two electrodes (sensor contacts)provided on the measurement gas side of the solid electrolyte. Thisconstruction makes possible a voltammetric measurement of two gascomponents in a gas mixture.

In addition, a gas sensor of the generic type is known from Germanpublished patent application DE 36 10 366 A1, in which a plurality ofelectrochemical measuring cells are arranged on a tubular support. Thisdevice allows only gaseous pollutants to be measured (not oxygen). Theevaluation of the measurement signals takes place based on thecharacteristics of the pollutant concentrations.

Furthermore, a gas sensor of this type is known from German Patent DE 4109 516 C2. In this device, the solid electrolyte is constructed in theshape of a platelet, on one side of which an electrode is applied whichfunctions as a reference electrode, and on the opposite side of which atleast two measuring electrodes are applied, which interact with variouscomponents of a gas mixture. The platelet-shaped sensor is built into ahousing, which is then to be installed as a gas probe in the exhaust gasduct of a motor vehicle, more specifically perpendicular to the flowdirection of the exhaust gas.

This probe functions without a reference gas, which is required forobtaining an electrode potential independent of the environment.However, such electrodes are not stable with respect to theirelectrochemical potential, especially when the mixture compositionchanges from lean to rich. In addition, with configurations of thesensor design which are not rotationally symmetric, it is very hard torealize a stable and uniform temperature distribution over the entiresurface. A similar, relatively complicated sensor is also known fromGerman published patent application DE4243 734 A1.

SUMMARY OF THE INVENTION

In view of the above, an object of the present invention is to provide agas sensor with which at least two gaseous components can be reliablydetected simultaneously over a wide range of gas mixtures, and whichalso ensures a stable reference signal with the aid of ambient air,which additionally and, if necessary, allows the influence of the oxygenconcentration by adding or removing oxygen at the respective measuringelectrodes.

These objectives are achieved according to the present invention,wherein the solid electrolyte is constructed with one side exposed tothe measurement gas and with a reference gas side separated from themeasurement gas. The arrangement of the electrodes with the referenceelectrode on the reference gas side and with the at least two measuringelectrodes on the measuring gas side is constructed such that one of thereference electrodes is assigned to at least one measuring electrode,which forms the anode of this electrode pair. The electrode pair isadapted to apply a voltage or a current for pumping of oxygen, and thearrangement simultaneously transmits at least two measurement signals,which correspond to different gaseous components. Alternatively, atleast one of the reference electrodes can be assigned to at least twomeasuring electrodes, which are arranged spaced from one another on thesame solid electrolyte.

According to the measurement method of the invention, oxygen is pumpedfrom the reference gas side to the measurement gas side, whereby anoxygen excess is formed on the measurement gas side, and a differencesignal is measured between two different measuring electrodes. Otheradvantageous amplifications of the invention, as well as the use of thegas sensor of invention, are described below and set forth in thedependent claims.

Advantageously the solid electrolyte, generally provided as a solidelectrolyte body of virtually any desired shape, is constructed as asmall tube closed at one end, which has on its inner wall a referenceelectrode, positioned as close as possible to the closed end, and aplurality of electrodes arranged on the outer side, exposed to themeasurement gas. The solid electrolyte consists, e.g., of partially orfully stabilized ZrO₂ or of CeO₂. The arrangement of at least twoindependent measuring electrodes on the solid electrolyte guarantees thesimultaneous detection of at least two measurement signals whichcorrespond to at least two different gas components. Since a tubularsolid electrolyte with a circular cross-section is used, thedisturbances at an installation point perpendicular to the exhaust gasflow are thereby minimized, so that the measurement gas flows around thesensor in a relatively uniform manner. Accordingly, the gas componentsbeing measured arrive at the measuring electrodes practically without adelay, and the disturbing turbulence is avoided.

If the gas sensor is used at temperatures below 400° C., it isadvantageous to provide the sensor with a heating element. The heatingelement for this purpose can be applied as a heating conductor, likewiseon the outer side of the solid electrolyte, wherein, however, in orderto avoid a short circuit, an electrically insulating layer is arrangedbetween the heating conductor and the solid electrolyte.

Expediently, at least one of the electrodes used as a measuringelectrode on the outer side of the solid electrolyte tube, closed at oneend, is made of a catalytically active material, wherein differentmeasuring electrodes can have different catalytically active materials.Consequently, the at least one measuring electrode is particularlysuited for the potentiometric oxygen measurement according to theprinciple of a Nernst probe. In contrast, the second measuring electrodeis made of a catalytically inactive material. This electrode ispreferably used for detecting hydrocarbons.

It is advantageous if the surface of the measuring electrodes facing themeasurement gas is covered with a preferably porous diffusion layerwhich, for example, can be made of aluminum oxide, spinel, or magnesiumoxide, and which can have a different layer thickness over eachmeasuring electrode, in order to be able to influence the oxygenresidence time aimed at.

The reference electrodes assigned to the mutually spaced apart measuringelectrodes can be divided into mutually spaced apart partial referenceelectrodes.

By using different catalytically active electrode materials for theadjacent measuring electrodes, assigned to the same referenceelectrodes, a gas-symmetrical differential measurement can be conductedbetween two measuring electrodes, wherein the selectivity with respectto hydrocarbons can be improved, for example by the choice of the oxygenpressure at these electrodes. At the same time, cross-influences arisingthrough changing lambda are avoided. By the connection of a measuringelectrode as the anode with respect to the reference electrode assignedto it, the targeted amount of oxygen can be pumped from the referencegas side to the measurement gas side by the application of a voltage ora current.

As catalytically active materials platinum or platinum alloys haveproven satisfactory. Furthermore, rhodium or palladium are also suitableas catalytically active electrode materials. For the catalyticallyinactive materials, which should be used for the second measuringelectrode, gold and gold alloys, as well as metal oxides have provensatisfactory. The catalytically inactive metal oxides are exemplified bymixed-conductivity perovskite compounds of the general formulaLn_(1−z)A_(1−x)B_(x)O₃, wherein Ln is a lanthanide cation, A is anelement selected from the group Mn, Cr, Co, Fe, Ti, or Ni (preferably Cror Ti), and B is an element selected from the group Ga, Al, Sc, Mg, orCa.

By varying the composition of the measuring electrodes, it is possibleto bring various gas components to the electrodes for interaction. Thegas sensor according to the invention is therefore particularly suitablefor applications for simultaneous measurement in gas mixtures whereoxygen or lambda and hydrocarbons or carbon monoxide are to be found.

The measurement of oxygen is preferably carried out at the catalyticallyactive measuring electrode, wherein the potential which arises,dependent on the concentration of oxygen in the measurement gas, ismeasured against the air reference electrode. This measuring electrodesets the measurement gas into equilibrium, and the voltage detected atthis electrode gives a signal corresponding to that of known lambdaprobes.

At the measuring electrodes, which are preferably made of acatalytically inactive material, such as mixed-conductivity metal oxides(mixed-oxides), a voltage can likewise be detected against the (air)reference, which is determined from the concentration of unburnedcomponents, i.e., from the concentration of hydrocarbons or carbonmonoxide in the exhaust gas.

In the method according to the invention, oxygen is pumped from thereference gas side to the side is exposed to the measurement, gas and adifference signal is measured between two different measuringelectrodes.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiment(s) which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1 is a set of characteristic curves of the measurement signals froma gas sensor of the invention according to FIG. 3, with a propane/oxygenmeasurement gas mixture;

FIG. 2 is a sectional view through a tubular gas sensor of the inventionfor amperometric oxygen measurement and for potentiometric hydrocarbonmeasurement;

FIG. 3 is a sectional view through another embodiment of a tubular gassensor of the invention for measurement of oxygen and hydrocarbons;

FIG. 4 is a sectional view through a tubular gas sensor of the inventionfor potentiometric measurement of oxygen and for amperometricmeasurement of oxygen and hydrocarbons.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows, by way of example, a set of characteristic curves ofmeasurement signals obtained with the aid of a gas sensor of theinvention. The sensor voltage in millivolts is plotted versus thepropane gas concentration in volume %. Air is used as the reference gas.Curve 1 shows a measurement signal of the catalytically active platinumelectrode; curve 2 represents a measurement signal of the catalyticallyinactive electrode (e.g., of gold); and curve 3 represents a differencesignal between the two electrodes in a gas-symmetrical system.

In FIG. 2 a sectional view is shown through a sensor according to theinvention. A catalytically active electrode 2″ is arranged on a tubularsolid electrolyte 1 of ZrO₂, and opposite to it the reference electrode9′ is installed in the reference air passage 4. Further, an insulatinglayer 8 of Al₂O₃ is arranged on the outer side of the ZrO₂ tube 1, onwhich a heating element 5 is arranged symmetrically for a rapid heatingof this gas sensor. Opposite to the measuring electrode 2′ on the solidelectrolyte 1 there are mounted two different catalytically activemeasuring electrodes 2, 2 a of platinum or a platinum alloy, which areassigned to the reference electrode 9. Both measuring electrodes 2, 2 a,for their part, are covered by a protective layer or a porous diffusionlayer 11 of aluminum oxide. Aluminum oxide is provided as an insulatingmaterial 12 between the measuring electrodes 2 and 2 a.

With a surplus of oxygen, which arises by application of a voltageU_(P1) between the electrodes 9 and 2 a or U_(P2) between the electrodes9 and 2 and the thereby-generated pumping of oxygen into the measurementgas space, this construction of the gas sensor allows the hydrocarbonconcentration or the carbon monoxide concentration to be determinedgas-symmetrically or potentiometrically at two external electrodes 2, 2a. In this case, electrode pairs 9, 2 and 9, 2 a can be coordinated toone another in an optimum manner. The high sensitivity achieved therebycan be influenced in a targeted manner by the heating elements 5, whichare separately assigned to the electrodes 2, 2 a. In the case of adivided construction of the reference electrode 9 as two partialreference electrodes, the sensitivity can be further increased. Thesolid electrolyte 1 is, for example, mounted in a housing in a mannerwell known to those skilled in the art, wherein the individual layersand electrodes can be electrically contacted, likewise in a knownmanner, for example at an end of the solid electrolyte 1.

Hydrocarbons are measured gas-symmetrically as a difference signal U₂between the electrodes 2 and 2 a, while between the electrodes 9′ and 2″the voltage U₁, is measured as the oxygen signal with respect to thereference.

FIG. 3 shows, likewise in section, a tubular solid electrolyte 1 ofZrO₂. A catalytically inactive electrode 3, made of a perovskitematerial, and catalytically active electrodes 2, 2 a, made of platinumor a platinum alloy, are applied on the outer side. The latterelectrodes are covered with a porous diffusion layer 11 of Al₂O₃. Thisporous diffusion layer can have different thicknesses over the twoelectrodes 2, 2 a. A reference electrode 9, 9′ or counter-electrode 9′is arranged in the interior of the tube 1, respectively opposite the twomeasuring electrodes 2, 3. Similarly to FIG. 2, the gas sensor isprovided with a heating element 5. The oxygen determination in thisembodiment is carried out amperometrically by means of a pump currentbetween the measuring electrode 2 and the reference electrode 9, whichfunctions here as the counter-electrode. The hydrocarbon determinationis carried out potentiometrically by measuring the voltage U₃ at theelectrodes 9′ and 3 or by the differential measurement U₂ between theelectrodes 2 and 2 a.

FIG. 4 shows a sensor construction, similar to that depicted in FIG. 2,but additionally, further measuring electrodes 2′, 2 a′ are arrangedover the diffusion layer 11. Here, the diffusion layer is alsoconstructed as a solid electrolyte, so that an amperometric measurementwith the pump current can be carried out between the measuringelectrodes 2 and 9 or 2 a and 9. The hydrocarbon determination iscarried out by the differential measurement U₂ between the electrodes 2′and 2 a′. The oxygen signal (potentiometric lambda determination) U₁ ismeasured between the electrodes 9′ and 2″. The two external electrodes2′, 2 a′ are covered with a porous oxygen ion conducting material 13,which also extends over the intermediate space between these twoelectrodes 2′, 2 a′. The material 13 is covered by a gas-tight sealinglayer 14, at least laterally until it extends onto the insulating layer8, in order to rule out leakage of the measurement gas or oxygen.

It will be appreciated by those skilled in the art that changes could bemade to the embodiment(s) described above without departing from thebroad inventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiment(s) disclosed, butit is intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim:
 1. A method of measuring oxygen and/or the air-to-fuel lambdaratio and hydrocarbons and/or carbon monoxide in gas mixtures using agas sensor, the gas sensor comprising at least one reference electroderepresenting a constant oxygen partial pressure, an oxygenion-conducting solid electrolyte, and at least two measuring electrodes,the measuring electrodes and the at least one reference electrode beingmounted directly on the oxygen ion-conducting solid electrolyte andhaving electrical leads for connection and for transmission ofelectrical measurement signals, wherein the oxygen ion-conducting solidelectrolyte has a measurement gas side exposed to the gas mixture and areference gas side separated from the gas mixture, and wherein a systemof the electrodes has the at least one reference electrode arranged onthe reference gas side and the at least two measuring electrodesarranged on the measurement gas side, and the system is so constructedthat the at least one reference electrode is assigned to at least twomeasuring electrodes of different catalytic activity, which are arrangedmutually spaced apart on the same oxygen ion-conducting solidelectrolyte and which are covered with a porous diffusion layer, whereinthe system simultaneously transmits at least two measurement signals,which correspond to different gaseous components of the gas mixture, themethod comprising the steps of: pumping oxygen from the at least onereference electrode to the two covered measuring electrodes; creating anoxygen surplus at the two covered measuring electrodes; and measuring adifference voltage signal between the two covered measuring electrodes,which is dependent on the content of hydrocarbons and/or carbon monoxidein the gas mixture.
 2. A method of measuring oxygen and/or air-to-fuellambda ratio and hydrocarbons and/or carbon monoxide in gas mixturesusing a gas sensor, the gas sensor comprising at least one referenceelectrode representing a constant oxygen partial pressure, an oxygenion-conducting solid electrolyte, and at least two measuring electrodes,the measuring electrodes and the at least one reference electrode beingmounted directly on the oxygen ion conducting solid electrolyte andhaving electrical leads for connection and for transmission ofelectrical measurement signals, wherein the oxygen ion-conducting solidelectrolyte has a measurement gas side exposed to the gas mixture and areference gas side separated from the gas mixture, and wherein a systemof the electrodes has the at least one reference electrode arranged onthe reference gas side and the at least two measuring electrodesarranged on the measurement gas side, and the system is so constructedthat the at least one reference electrode is assigned to at least twomeasuring electrodes, which are arranged mutually spaced apart on thesame oxygen ion-conducting solid electrolyte and which are each coveredwith a porous diffusion layer of different thickness, wherein the systemsimultaneously transmits at least two measurement signals, whichcorrespond to different gaseous components of the gas mixture, themethod comprising the steps of: pumping oxygen from the at least onereference electrode to the two covered measuring electrodes; creating anoxygen surplus at the two covered measuring electrodes; and measuring adifference voltage signal between the two covered measuring electrodes,which is dependent on the content of hydrocarbons and/or carbon monoxidein the gas mixture.